1. Background of the First Invention
In order to reduce the production costs of structural members, such as mechanical component parts, it is possible to think of utilizing iron-based sintered-alloy members, which are sintered by heating powder compacts which are made by press forming raw material powders in which iron is adapted to be the major component. When using iron-based sintered-alloy members, it becomes possible to obtain products (sintered bodies), which are close to the final configurations, and accordingly it is possible to intend the reduction of the production costs or material costs of structural members by means of machining reduction or material-yield improvement, and the like. In order to do so, the strength and before/after-sintering dimensional stability of iron-based sintered-alloy members become important.
From such viewpoints, Fe—Cu—C-system iron-based sintered alloys, which are made by sintering powder compacts composed of Fe—Cu—C-composition raw material powders, have been used heavily. It is because Cu is an element, which is effective for the strength improvement and before/after-sintering dimensional stableness of iron-based sintered alloys. Therefore, contrary to general iron/steel alloys, in the case of iron-based sintered alloys, Cu has been considered its essential component virtually.
2. Background of the Second Invention
In order to reduce the production costs of structural members, such as mechanical component parts, it is possible to think of utilizing iron-based sintered-alloy members, which are sintered by heating powder compacts which are made by press forming raw material powders in which iron is adapted to be the major component. When using iron-based sintered-alloy members, it becomes possible to obtain products (sintered bodies), which are close to the final configurations, and accordingly it is possible to intend the reduction of the production costs or material costs of structural members by means of the reduction of machining or material-yield improvement, and the like. In order to do so, the strength and before/after-sintering dimensional stability of iron-based sintered-alloy members become important. As measures therefor, the following methods have been adopted.
First of all, it is to employ Fe—Cu—C-system-composition raw material powders. It is because Cu is an element, which is effective for the strength improvement and before/after-sintering dimensional stableness of iron-based sintered alloys. However, even when raw material powders with such compositions are employed, if the density of sintered bodies is low, no high-strength sintered bodies are desired after all. Moreover, iron-based sintered-alloy members, which include Cu, are not preferable in view of costs, in view of recycling, and the like, as well.
As a method of heightening the density of sintered bodies, it is possible to think of a method of carrying out double-pressing and double-sintering (2P2S). However, since this method carries out pressing and forming repetitively, it causes the cost rise of sintered bodies so that it is not preferable. A powder forging method, in which sintered bodies are forged to make them denser (highly densify them), is also available. However, in this method, since the dimensional accuracy of sintered bodies cannot be maintained, the advantages of powder sintering method have been sacrificed, making the costs of structural members lower which relies on iron-based sintered-alloy members, the original purpose, cannot be intended.    [Patent Literature No. 1] U.S. Pat. No. 6,346,133    [Patent Literature No. 2] U.S. Pat. No. 6,364,927    [Patent Literature No. 3] Japanese Patent Publication No. 3309970    [Patent Literature No. 4] Japanese Unexamined Patent Publication (KOKAI) No. 58-210147    [Patent Literature No. 5] Domestic Re-publication of PCT International Publication for Patent Applications No. 10-510007    [Non-patent Literature No. 1] High Strength Si—Mn-Alloyed Sintered Steels. P. M. vol. 17 Int. No. 1 (1985)    [Non-patent Literature No. 2] “Effect of Sinter-Hardening on the Properties of High Temperature Sintered PM Steels,” Advances in Powder Metallurgy & Particulate Materials, MPIF, 2002, part 13, pp. 1-13    [Non-patent Literature No. 3] “New focus on chromium may sidestep alloy cost increases,” MPR. September (2004), pp. 16-19